In Circuit Emulation Services over Packet (CESoP), it is a key factor that influences the system performance to accurately recover a time division multiplex (TDM) service clock. For example, if a dedicated leased line is used between two clients connected by a circuit emulation service channel of the operator, clock frequencies of TDM services of the clients must be recovered precisely at the egress of the packet-based network. Otherwise, long-time frequency mismatch will result in overflow or read-empty of waiting queue at the egress of the packet-based network and cause slip impairment.
An adaptive approach is a common method for recovering clock, in which a receiving end extracts timing information from timestamps carried in packets and a packet arrival interval. This approach needs no extra reference clock, and is very suitable for use in a packet-based network. The process for adaptive clock recovery is shown in FIG. 1. In FIG. 1, the function of service adaptation modules is to complete adaptation between a TDM stream and packets. The service adaptation module at a sending end obtains a clock signal from TDM device to drive a counter to count, encapsulates and sends data in packets when receiving a certain amount of TDM stream data, and meanwhile loads a counter value when the packets are sent, that is, a timestamp into a packet header. The service adaptation module at a receiving end receives the packets, uses the timestamp to recover a service clock at the sending end, and utilizes the clock to send the TDM data in the packet load in the form of a bit stream.
The clock recovery needs both timestamps at the packet sending and reception, which inevitably introduces packet delay characteristics of the packet-based network. The key of the adaptive approach lies in how to effectively filter out the impairment of a packet delay variance (PDV) on the clock recovery so as to precisely recover the clock signal at the sending end.
The prior art provides a method for reducing the impairment of the PDV on the clock recovery. The method is as follows: outputting a write signal after performing traffic shaping on a received data packet; determining a data amount buffered in a first-in-first-out buffer according to the write signal and a read clock output by a digital oscillator; computing a filter parameter according to the write signal and the determined buffered data amount; and determining the recovered read clock according to the filter parameter.
In the implementation of the present disclosure, the inventors find that at least the following problems exist in the prior art.
The prior art reduces the impairment of the PDV on the clock recovery mainly by filtering. However, the change of timestamp intervals at the receiving end may be caused by various reasons, so that the filtering method is hardly suitable for different situations. Especially when a certain packet experiences a large delay in the network, a receiver timestamp (RT) is mutated, which seriously impairs the clock recovery.